1. Field of the Invention
The invention relates generally to subframes for heavy-duty vehicles, such as tractor-trailers, and in particular to movable subframes for tractor-trailers. More particularly, the invention is directed to a movable subframe for tractor-trailers, which includes a pair of main members formed with openings for receiving pins of a retractable pin mechanism, for locking the movable subframe in a selected longitudinal position relative to the tractor-trailer body. Still more particularly, the invention relates to a slider pin assembly for a movable subframe for tractor-trailers in which the slider pin assembly includes integral inboard and outboard stops that eliminate the prior art press-fit stop pin, enables easy installation and repair of the slider pin assembly, reduces weight and material cost of the slider pin assembly, and increases performance, durability and life-span of the slider pin assembly and its components.
2. Background Art
Movable subframes, typically referred to as slider boxes, slider subframes, slider undercarriages, or slider secondary frames, have been utilized on heavy-duty vehicles such as tractor-trailers or semi-trailers for many years. One or more axle/suspension systems usually are suspended from a single slider box. For purposes of clarity, hereinafter the present invention will be referred to as a slider box. It is understood that a slider box outfitted with usually two axle/suspension systems typically is referred to as a slider or slider tandem, and again, for purposes of clarity will hereinafter be referred to as a slider tandem. Of course, as mentioned above, a slider box may also be outfitted with a single axle/suspension system, or three or more axle/suspension systems. Reference herein shall be made to a slider tandem by way of example, with the understanding that such reference applies to a slider outfitted with one, two, three or more axle/suspension systems. The slider tandem in turn is mounted on the underside of the trailer frame, and is movable longitudinally therealong to provide a means for variable load distribution and vehicle maneuverability.
More specifically, the amount of cargo that a trailer may carry is governed by local, state and/or national road and bridge laws, and is dependent on proper load distribution. The basic principle behind most road and bridge laws is to limit the maximum load that a vehicle may carry, as well as limit the maximum load that can be supported by individual axles. A trailer having a slider tandem gains an advantage with respect to laws governing maximum axle loads. More particularly, proper placement of the slider tandem varies individual axle loads or redistributes the trailer load so that it is within legal limits.
Once properly positioned, the slider tandem is locked in place on the underside of the trailer by a retractable pin mechanism. The retractable pin mechanism generally includes two or more, and typically four, pin assemblies, which may be interconnected by a crank or other retraction mechanism. When the pins are in their extended position, they each extend through a respective opening formed in the main members of the slider box and a selected aligned one of a plurality of openings formed in rails of the trailer body. The pins thereby lock the slider box in a selected longitudinal position relative to the trailer body.
Prior art pin assemblies typically include a pin body that is connected to the retraction mechanism. The pin body extends through a pin guide that is attached to the slider box frame, which allows transverse movement of the pin body. A spring is disposed about the pin body with the inboard end of the spring biased against the pin guide. The outboard end of the spring is biased against a lip formed in the pin body. The pin outboard end is disposed through a pin collar that is attached to the slider box frame and which is aligned with an opening formed in the main member sidewall. The pin outboard end extends through the pin collar and the aligned main member opening when the slider pin assembly is in an extended state. When the retraction mechanism is operated and the pin is pulled inwardly against the bias of the spring, the pin outboard end is pulled inwardly through the main member opening so that it does not engage the aligned openings in the rails of the trailer body on which the slider box is operatively mounted.
These prior art pin assemblies, although they typically satisfactorily perform their intended function, are heavy. Also, because they retract so that the spring is fully collapsed against its bias, they can be prone to spring fatigue and bell mouthing issues exhibited in the spring. More specifically, because the prior art slider pin assembly completely collapses the spring against its bias and imparts significant force to the fully collapsed spring in the direction of the collapse, the outboard end of the spring can potentially exhibit bell mouthing, or deformation of the spring at its outboard end, as it is forced against the shoulder of the pin body. More particularly, bell mouthing is the increased diameter of the outboard-most coils of the spring that result from a buckling load which can potentially cause the outboard spring coils to be refracted or pushed over adjacent spring coils effectively reducing both the length and the stiffness of the spring. This in turn can compromise the spring's ability to properly bias the pin body into its extended state when the retraction mechanism is de-activated or released by the operator of the vehicle. Moreover, because the stop pin of the prior art slider pin assembly must be press-fit into the pin body opening as described above, the slider pin assembly is difficult to install and remove from the slider box, making field installation and repairs of the slider pin assembly difficult. Additionally, when using a coating process for the slider box, such as galvanizing, the process does not allow the slider pin assembly to be installed prior to welding the slider box together and performing the galvanizing process, and thus installation of the prior art slider pin assembly following the coating process requires significantly more production time.
Therefore, a need exists in the art for a slider pin assembly that overcomes the problems associated with prior art slider pin assemblies. The slider pin assembly of the present invention includes integral inboard and outboard stops that reduce spring fatigue and spring bell-mouthing issues exhibited by prior art slider pin assemblies during operation of the slider pin assembly. Moreover, the slider pin assembly of the present invention is lightweight, reducing the material weight of each slider pin assembly by as much as about 1.14-1.22 lbs. In addition, the slider pin assembly of the present invention reduces material costs and eliminates the press-fit stop pin of prior art slider pin assemblies, which allows easier installation and removal of the slider pin assembly. Thus field installation, removal, maintenance and repair of the slider pin assembly is significantly easier. Additionally, when using a coating process for the slider box, such as galvanizing, that requires the pins to be installed after welding and galvanizing the slider box, installation of the slider pin assembly of the present invention requires significantly less production time than prior art slider pin assemblies. Furthermore, the slider pin assembly of the present invention increases performance, durability and the life span of the slider pin assembly and its components.